For many applications, and particularly for the practice of high pressure liquid chromatography, there is required a fluid pumping system which can accurately and reproducibly pump metered amounts of fluids into a high pressure load. It is important that the fluid flow be precisely controlled independently of the pressure of the load. Particularly if a chromatography system is to utilize gradient elution in which different fluids are to be mixed, the amount of each fluid must be precisely measurable at any given load pressure.
In the prior art, there are known different fluid delivery systems for liquid chromatography. It has been a problem with the prior art devices that the compressibility of the fluid and the mechanical compliance of the pump combine to cause a severe drop in flow rate as the load pressure increases, this phenomenon being commonly referred to as "roll off." To accurately meter the fluid flow, some presently known delivery systems employ a flow transducer which generates electrical feedback to control the pumping rate of the mechanical pump. However, these sensors typically depend on a measurement of differential pressure drop across a flow restrictor. Since the pressure drop depends on liquid viscosity which is a function of temperature, the flow sensor must be precisely maintained at a constant temperature; this is inconvenient and expensive. Furthermore, if a small particle or other residue lodges in the restricted area, a change in pressure reading results. Additionally, recalibration of flow sensors is necessary when a different fluid is being pumped.